Fluid flow cells incorporating ion selective electrodes arranged to measure the ionic activity of fluids flowing through such cells are well known in the art. For example, the System E4A.TM. Electrolyte Analyzer available from Beckman Instruments, Inc., Brea, Calif. 92621 incorporates such a fluid flow cell. In simplified form, the flow cell includes a body through which are formed a reference solution conduit and sample fluid conduit. The reference solution conduit conducts a reference solution past an electrochemically active surface of a compensating electrode. Similarly, the sample fluid conduit conducts a sample fluid stream past electrochemically active surfaces of respective ion selective electrodes. A low impedance salt bridge near the compensating electrode electrically connects the reference solution conduit to the sample fluid conduit.
The ion selective electrodes in the sample fluid conduit measure the concentrations of ions in the sample fluid in proportion to the concentration of certain electrolytes, such as, for example, sodium, potassium, and chloride. The purpose of the compensating electrode is to provide a compensating signal which varies with respect to environmental changes and influences, such as temperature changes and electrical noise, to which all of the ion selective electrodes are subjected.
The E4A flow cell further includes an acid diluent conduit which intersects the sample fluid conduit on the downstream side of the salt bridge so as to combine the sample fluid with an acid diluent. The resulting diluted sample mixture flows past another electrode which is one of a pair used to measure CO.sub.2 concentrations.
The E4A flow cell is part of a fluid system within the analyzer that includes fluid reservoirs, fluid conduits, pumps and pinch valves. Unfortunately, the fluid paths defined by the fluid system leading to and from the flow cell act as antennas, causing potentially interferring electrical signals induced in such paths to be conducted into the flow cell and detected by the ion selective electrodes. In order to reduce such interference, the E4A analyzer includes electrostatic or Faraday shielding around the cell and a portion of the connecting fluid paths as well as grounding bypass capacitors connected to the outlets of the fluid reservoirs. Also, a solution ground, place at the flow cell entry port of the reference solution conduit, serves in part to ground noise which may enter the flow cell.
Although the E4A analyzer interference reduction techniques just described have proven to be sufficient, such techniques have been found to be ineffective or overly burdensome where the E4A flow cell is used in a physically larger instrument. For example, Faraday shielding becomes very difficult or complicated where fluid reservoirs are located at greater distances from the flow cell as compared to the E4A or such reservoirs are larger than those used on the E4A. Electrical noise sources, such as stepper motors and solenoids, which could be place outside the Faraday shielding on the E4A may be very difficult to adequately shield on a larger instrument. Furthermore, each grounding bypass capacitor at a respective fluid reservoir becomes less effective as the associated fluid path is lengthened and/or interrupted by pinch valves or peristaltic pumps.
Thus, there is a need for an improved flow cell of the type described above which is less susceptible to noise entering the cell via the various fluid conduits connected thereto.